[epiar-devel] Virtual framerate + interpolated graphics demo

[Christopher Thielen]

Just for education and knowledge's sake, attached is the result of 
extracting the important bits from Dan Olfson's "fixed frame rate pig" 
demo which I will use in the coming days to fix up Epiar's timer and 
drawing speed issues.

The demo is simple: a box bounces up and down the screen. Pressing 'i' 
toggles the interpolation (it starts out on). The interpolation effect 
is subtle and is more noticeable when your sprite coordinates do not 
fall on integers. It also works better at certain speeds, e.g. very slow 
and very fast don't matter because integers tend to be within a few 
percetange points of being correct, but those important in-between 
speeds look a little smoother.

The important aspect of this however is the implementation of a virtual 
frame rate. The game becomes locked at a certain virtual frame rate, 
e.g. 50 Hz for a game like Epiar, in which the drawing loop is ignored 
giving the logic time to catch up; this is how a lot of modern games do 
it, and this removes the problem of the game literally running slower on 
slower computers, e.g. on a slower computer, things will still move at a 
constant rate unless it is _much_, _much_ slower than what we would 
expect, but only in the case where the drawing code and the update code 
take similar amounts of time, and they don't, it's not even close.

Noticeably, having a virtual frame rate means you can use hard-coded 
values in the code (you mustn't, fyi, but technically could) just like 
the old days where you were programming for one machine that always ran 
at the same speed and didn't have to worry about the code running on 
various machines with various speeds. tl;dr you don't need to calculate 
deltas for movement based on the timer anymore. HOWEVER, we will because 
this is proper and in case we ever switch timer behavior, we don't want 
to have to go back in and add them. Plus, it gives us a little added 
bonus in ensuring proper game logic speed and drawing positions across 
an even wider array of hardware.

- Chris
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